The invention is directed to an apparatus for digitally analyzing an electronic signal, and relates, more specifically, to a digital video analyzer for analyzing and visually displaying composite video waveforms of the type used in television broadcasting.
In the usual television video signal, one complete picture is called a frame which, in turn, consists of two interlaced fields. A frame comprises 525 horizontal lines in the U.S. NTSC color TV system (625 lines in the European PAL color system). In order to reduce flicker effects, two fields of 262.5 horizontal lines a piece are used to scan all 525 horizontal lines. The 262.5 horizontal lines scanned by the first field of one frame comprises a scanning of every other line. During the next field, the remaining 262.5 lines are scanned.
Associated with the scanning of each field is a portion of the video waveform which includes control signal for the period during which the electron beam is moved from the bottom of the screen to the top of the screen to begin scanning the next field. This interval comprises a plurality of vertical sync pulses, a plurality of equalization pulses, a plurality of horizontal sync pulses, and a vertical blanking signal. In the NTSC system, 60 fields per second are scanned; while in the PAL system, 50 fields per second are scanned.
Each horizontal line comprises a horizontal blanking interval, within which occur a horizontal sync signal, followed by a color synchronization burst and then luminance and chrominance information. In the NTSC system, the full horizontal line has a period of approximately 63.56 microseconds, with the horizontal blanking and sync signal occupying a period ranging from 10.16 to 11.43 microsecond. The vertical blanking and synchronization interval occupies 21 horizontal lines or 1250 to 1333 microseconds. Upon consideration of the above signals, frequencies and pulsewidths, it is apparent that a wide range of signals, frequencies and time periods are present, all of which need to be monitored in order to provide a satisfactory video signal.
In a typical television broadcast studio operation, a number of signal analyzing apparatus are required to monitor and analyze the video signal. Among these apparatuses are a picture monitor for viewing the actual picture content of the video signal, a field analyzer for displaying a full field of the composite video signal, an apparatus for viewing the signal characteristics of a selected single horizontal line, apparatus for detecting the chrominance signal within the selected line so that differential gain and phase measurements may be displayed, as well as a polar display of the chrominance difference signals obtained.
In the past, each piece of signal information sought to be monitored required a separate piece of test equipment. Necessarily, this requirement reduces the ability for simultaneous viewing of the various picture parameters with respect to each other by station personnel. Typically, special test circuits used in conjunction with a studio oscilloscope are often used to monitor such picture parameters as the chrominance vector display, differential phase and differential gain, as well as the selected horizontal line.
The field analyzer provided a visual display of a full field of the composite video signal. Typically, the field analyzer was an oscilloscope-type display of the composite video scanned at a field rate. The result was a near solid band of illumination across the oscilloscope screen with the vertical sync intervals appearing as non-illuminated columns, with those levels being present for a longer period having a higher intensity than those signal level present only for a short period within the waveform.
In the past, selected lines of the composite video signal could be viewed using an oscilloscope; however, because a single line of the video signal is not repeated until a full field later, the scope display appears as a number of horizontal line waveforms traced over each other. As such, clean display of a single horizontal line is difficult to obtain. Additionally, because the desired horizontal line waveform will be swept just once across the oscilloscope screen, the intensity of the displayed waveform will be low.
In a storage scope, a single horizontal line can be electrostatically preserved in a storage tube and displayed, however, the quality of the displayed waveform diminishes over time and can easily be obliterated by the wrong turn of a dial.
The polar or vector display of the chrominance different signals was typically obtained using a vector scope which demodulated the composite video signal to derive the difference signals and converted the difference signals into a polar display format.
A common characteristic of the above previous test equipment for monitoring the composite video signal is that their circuits and display signals were directed to the display of waveforms using an oscilloscope-type format, that is, deflecting a beam vertically as it is moved horizontally across the screen. In the past, some effort has been directed toward generating a signal format which is suitable for displaying a waveform using a raster-scanning type display. Typical of the patents directed to such apparatus for monitoring signal waveforms are Hess, et al., U.S. Pat. No. 4,145,706 and Schneider, U.S. Pat. No. 4,058,826. In Hess, the invention was directed to an apparatus for displaying a horizontal frequency-coupled input signal on the picture screen of a video display device such as domestic television receiver. With respect to composite video signal analysis, the above invention had limited breadth. As disclosed, the invention appears capable of displaying only the active line portion of the composite video signal. As such, the portion of the composite video signal such as horizontal sync and the colorburst synchronizing signal were not displayable.
In Schneider, an apparatus is disclosed for displaying a waveform on a raster-scanning type display. However, the manner in which the horizontal display signal is generated requires that in order to view the display signal in the usual vertical axis--amplitude and horizontal axis--time orientations, the raster-scanning display device must be turned on its side.
In none of the above apparatuses is it suggested or taught that substantially all of the various studio test equipment functions can be combined into a single digital video analyzing apparatus.
Additionally, even when a satisfactory waveform display can be obtained using an oscilloscope-type display, the time periods necessary to display such a waveform, for example, one full horizontal line, are too long for direct conversion of such waveform for display on a raster-scanning type display. For example, because a full horizontal line of a composite video signal occupies approximately 64 microseconds of time, and because the active line of a raster-scanning type display is typically 54 microseconds, it is clear that in order to display the 64-microsecond waveform on the raster-scanning display, some portion of that waveform will have to be omitted. This practical 54-microsecond time period limitation on the duration of waveform which can be displayed on the raster-scanning display further limits the utilization of a raster-scanning type display in the monitoring of the various waveforms of interest.
A further limitation of the prior test equipment apparatus was the single-color beam utilized to trace out a particular waveform on the visual display screen. If two or more waveforms were being monitored, the waveforms have to be physically separated on the screen so as to be distinguishable from each other.
The present invention of a digital video analyzer takes the place of separate test instruments by accepting up to three synchronized or non-synchronized video signals for simultaneous display on a precision color video monitor in three distinct colors.
Each signal can be independently stored in memory contained within the apparatus for later recall or for comparative evaluation, or as a signal source.
A reference graticule, which is digitally generated, is automatically matched to each test mode. This permits fast and accurate measurement, and eliminates the need for calibration.
The internal memory of the present invention permits spatial and time analysis of the video signal. Additionally, all waveform displays can be shown superimposed on the picture selected as the input source.